1. Field of the Invention
The present invention relates to a method and apparatus for dynamic braking, and more particularly to dynamic braking for the electric motors used in the electro-mechanical drive systems of a plastics processing machine.
2. Description of the Related Art
The increasing use of electrically driven mechanisms in plastics processing machinery requires careful attention to related changes which may affect the safe operation of the machines. For example, the large horsepower motors used to power the primary systems in an injection molding machine have significant inertia in the rotor of when operating. When it is necessary to decelerate a motor and the associated load rapidly, such as an emergency stop of an operating mechanism, the motor acts like a generator and the electric energy produced by the motor must be dissipated.
Emergency braking systems for electric motors are well known in the prior art and are generally referred to as dynamic braking systems. Such systems typically use resistive dampers which are connected to the motor windings by a switch mechanism that is activated when an emergency stop is necessary. The stored energy in the motor windings is dissipated as heat by the specially provided resistors, applying a braking effect to the rotor of the motor. FIGS. 3a and 3b show a typical circuit for connection of dynamic braking resistors to an electric motor; FIG. 3a is a schematic showing the delta connection, while FIG. 3b shows a wiring diagram.
The resistors required for the dynamic braking circuit are dedicated for this single purpose and, typically, must be relatively large in physical size, even though their usage is very limited. Given the cost of these resistors and the amount of space they occupy in the electrical cabinet of a plastics processing machine, it would be advantageous, if possible, to eliminate these special resistors included solely for the dynamic braking function.
In electrically driven injection molding machines, for example, there may be as many as three or more high horsepower electric motors which all require electrical systems to provide dynamic braking, as described above. Although various dynamic braking systems are currently available for the motors used in this type of application, there has not been a system which avoids the negative aspects of size and cost associated with a dedicated resistor bank.